Oxygen and nitrogen are produced commercially in large quantities by the cryogenic rectification of feed air, such as in a double column system wherein the product is taken from the lower pressure column. At times it may be desirable to produce the product at a pressure which exceeds its pressure when taken from the lower pressure column. In such instances, gaseous oxygen may be compressed to the desired pressure. However, it is generally preferable for capital cost purposes to remove the product as liquid from the lower pressure column, pump it to a higher pressure, and then vaporize the pressurized liquid to produce the desired elevated pressure product gas.
In such a system, generally termed a product boiling system, the liquid is vaporized in a product boiler by indirect heat exchange with a condensing fluid, typically pressurized feed air. The resulting liquid feed air is then passed into the cryogenic air separation plant for separation. Two liquid feed air arrangements are known. In one arrangement, all of the liquid feed air is passed into the higher pressure column wherein it undergoes cryogenic rectification. In another arrangement, the liquid feed air is divided into a first portion, which is passed into the higher pressure column, and into a second portion which is passed into the lower pressure column. The latter arrangement is preferred because it enables the cryogenic rectification plant to operate more efficiently due to judicious distribution of the incoming liquid feed air among the columns.
The cryogenic rectification of feed air, especially for the production of elevated pressure gaseous product, is an energy intensive operation and any improvement in energy efficiency is desirable.
Accordingly, it is an object of this invention to provide a system for the cryogenic rectification of feed air wherein at least some of the feed air is liquefied prior to entering the column or columns of the cryogenic air separation plant, which has improved efficiency over heretofore available such systems.